1. Field of the Invention
The present invention is directed to the field of digital data communication and more specifically to the area involving circuitry that functions as a buffer between input/output ports of microcomputers and associated data links.
2. Description of the Prior Art
Many microcomputer applications require external interface circuits to protect the microcomputer or enhance its drive capabilities by supplying the currents and voltages necessary to provide output to an associated data communication bus. Prior art interface circuits are typically designed to drive an associated external data bus in response to the data signals output from the microcomputer or to receive input date from the associated data communication bus and supply that data to the microcomputer.
A typical bi-directional interface circuit is shown in U.S. Pat. No. 4,471,243 which utilizes a single bi-directional data bus connection between the microprocessor and the external interface circuit. In addition, that interface circuit is connected to the microprocessor via two other control lines in which the microprocessor provides control signals to the interface circuit. The control signals set the interface circuit in either a receive or transmit mode so that data may be respectively received from an external source and conveyed to the microprocessor or transmitted by the microprocessor to the external load.
U.S. Pat. No. 3,769,525 to Foss et al is directed to an external bi-directional amplifying circuit which also uses a control line in addition to the bi-directional data line in order to control the functional gating of the circuit.